disks - Properties of Spiral Galaxies II Kinematics of...

Properties of Spiral Galaxies IIKinematics of DisksAs in all spiral galaxies, everything in our Galaxy orbits around the Galactic centerDifferential rotation - material closer to the center travels on faster orbits (takes less time to make one full orbit)Similar to the way the planets orbit the SunOrbital periods at different distances from GC tell us the distribution of mass in the Galaxy

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Local Standard of Rest (LSR) - reference frame for measuring velocities in the Galaxy. This would be the position of the Sun if its motion were completely governed by orbital motion around the GalaxyThe Sun (and most stars) are on slightly perturbed orbits.Sun motion wrt to LSR is determined by looking at average motions of all stars in the Sun’s vicinity.V = Vy (velocity in direction of galaxy rotation) U = Vx(velocity towards GC) = 10 km/sW = Vz(velocity towards NGP) = 7.2 km/s•V depends on color – stellar pops•asymmetric drift – mean Vrot of stellar pop lags behind LSR more and more with increasing σ of stars. •Comparing sun to this reference frame produces V which grows with lag •Fit to data gives at S2= 0, V = 5.2 km/s

With respect to the LSR, the Sun is moving at about ~14 - 20 km/s towards RA=18h Dec=30 degrees and lies about 10-20 pc above the Galactic plane. Position and Velocity of LSR in Galaxy(adopted in 1985 by IAU; based on globular cluster positions)Ro = 8.5 kpc Vo = 220 km/sMore recently, values of 8 kpc and 200 km/s have been estimated (see SG 2.3 and also Eisenhauer et al. 2003)

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Back to the board to derive Oort constants – understanding the effects of differential rotation near the sun.

Determination of Rotation Curve of the Milky Way Determined from 21-cm line observations Assume circular orbits and that there is at least some Hydrogen all along any given line-of-sight Especially important to have measure of gas at subcentral point

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